Method for providing conductive composite particles

Abstract

[Problem] To provide: a novel conductive material that is in a dried powder form effective in reducing environmental impact and improving long-term stability and, at the same time, has a high dispersibility and uniformity in an electrode coating that achieves an improvement in battery performance; and a method for producing said conductive material. [Solution] Conductive material composite particles containing at least a conductive material and a dispersant, characterised by: the particle size distribution D50 and sieved particle size of the particles being 20 µm or more and 150 µm or less, respectively; the DBP oil absorption of the conductive material being 550 ml / 100 g or less; and containing 1-10 parts by weight of the dispersant in relation to 100 parts by weight of the conductive material.

Description

Technical Field

[0001] The present invention relates to a method for producing conductive composite particles that have high dispersibility when mixed with an electrode active material and a binder (adhesive), have high uniformity in an electrode coating film, do not contain coarse particles or foreign substances that can cause a short circuit during operation, and are suitable for use in power storage devices such as batteries and capacitors after removing dispersion media such as water and organic solvents.

Background Art

[0002] In current lithium-ion secondary batteries, positive and negative electrodes in which an electrode active material is coated on a strip-shaped metal foil are used. After these are wound together with a separator, they are housed in a battery can. Among these, a lithium transition metal composite oxide or the like is used as the electrode active material for the positive electrode. These electrode active materials used alone in the positive electrode have poor electron conductivity, that is, poor conductivity. Therefore, to impart conductivity, a highly structured conductive carbon black or a carbon material such as graphite that exhibits significant anisotropy in crystals is added as a conductive material, and dispersed in a non-aqueous solvent such as N-methyl-2-pyrrolidone together with a binder to prepare an electrode paste. This electrode paste is applied and dried on a metal foil to form an electrode coating film, and a positive electrode is prepared. In the negative electrode, in addition to materials with low conductivity such as silicon, carbon-based materials such as graphite are mainly used as electrode active materials. Although carbon-based materials have conductivity, depending on their size, gaps may occur between the active materials and they may not be able to fully form an electron conduction path alone. Therefore, it is effective to improve the conductivity using a conductive material as in the case of the positive electrode. Thus, the conductive material plays an important role in lithium-ion secondary batteries. To improve battery performance such as high charge-discharge capacity and long-term stability of charge-discharge ability, a conductive material with high dispersibility and uniformity that can be easily dispersed and evenly impart conductivity around the active material is required.

[0003] However, conductive carbon materials, which are mainly used as conductive materials, are fine powders with small primary particle sizes and strong cohesive forces, making uniform dispersion extremely difficult. Therefore, when mixing them with materials such as electrode active materials to prepare electrode paste, if the cohesiveness is not sufficiently broken up by prolonged stirring, the conductive material will be unevenly distributed within the electrode coating. As a result, areas with localized poor conductivity occur within the positive electrode plate, preventing sufficient electron movement and thus preventing the effective utilization of the electrode active material, which has been cited as a drawback that results in low charge and discharge capacity (Patent Document 1).

[0004] Therefore, conductive material dispersion pastes have been proposed that improve dispersibility by pre-dispersing the conductive material in a solvent to create a liquid or paste-like conductive material (Patent Documents 2-10, etc.). Furthermore, with the aim of reducing environmental impact, a technology has been proposed in which conductive material fine particles are produced by removing the solvent from the conductive material dispersion paste by drying after production, thereby reducing the amount of organic solvent used, and lithium-ion secondary batteries are produced using this powder (Patent Document 11). In addition, Patent Document 12 proposes a carbon black composition obtained by filtering and drying the prepared conductive material dispersion paste to remove the solvent, then crushing it in an agate mortar and pestle, and passing it through a sieve with an aperture diameter of 45 μm, as well as a lithium-ion secondary battery electrode using this carbon black composition. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2003-308845 [Patent Document 2] Japanese Patent Publication No. 2011-70908 [Patent Document 3] Japanese Patent Publication No. 2011-113821 [Patent Document 4] Patent No. 4235788 [Patent Document 5] Japanese Patent Publication No. 2010-238575 [Patent Document 6] Japanese Patent Publication No. 2011-192020 [Patent Document 7] Japanese Patent Publication No. 2007-335175 [Patent Document 8] Japanese Patent Publication No. 2004-281096 [Patent Document 9] Japanese Patent Publication No. 2009-252683 [Patent Document 10] WO2014 / 042266 [Patent Document 11] Japanese Patent Publication No. 2012-9227 [Patent Document 12] Japanese Patent Publication No. 2018-62545 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] However, technologies for producing conductive material dispersion pastes, such as those described in Patent Documents 2 to 10, have drawbacks, including the need to use large amounts of environmentally harmful organic solvents during manufacturing, and a lack of long-term stability due to sedimentation and re-aggregation. Furthermore, while the technology disclosed in Patent Document 11 succeeds in reducing environmental impact, improving long-term stability, and handling aspects such as storage and transportation by using a powder with the solvent removed, the uniformity of the conductive material within the electrode paste and electrode coating is insufficient. As a result, the electron conductivity imparted to the active material is insufficient, and its capabilities are not fully utilized, thus limiting the effect on improving battery performance. The carbon black composition in Patent Document 12 also has insufficient electron conductivity imparted to the active material, and the inventors' investigations have revealed that lithium-ion secondary batteries using this carbon black composition do not have sufficient battery performance.

[0007] The objective of the present invention is to resolve the problems found in the prior art and overcome those challenges. Specifically, it aims to provide a new conductive material and a method for producing the same, which is in a dry powder form that is effective in reducing environmental impact and improving long-term stability, while possessing high dispersibility and uniformity in an electrode coating that improves battery performance. [Means for solving the problem]

[0008] As a result of diligent research, the inventors of the present invention have found that the above problems can be solved by using conductive composite particles that contain at least a conductive material and a dispersant and exhibit predetermined physical properties, leading to the present invention.

[0009] In other words, the present invention is (1) Conductive composite particles characterized by containing at least a conductive material and a dispersant, having a particle size distribution D50 of 15 μm or more and a particle diameter of 150 μm or less determined by sieving, having a DBP oil absorption amount of the conductive material of 550 ml / 100 g or less, and containing 1 to 10 parts by weight of dispersant per 100 parts by weight of conductive material. (2) Conductive composite particles characterized by containing at least a conductive material and a dispersant, wherein the particle size distribution D50 is 15 μm or more and the upper limit of the particle size is 300 μm or less, the DBP oil absorption amount of the conductive material is 550 ml / 100 g or less, and the dispersant is contained in 1 to 10 parts by weight per 100 parts by weight of conductive material. (3) Conductive composite particles according to either (1) or (2) above, characterized in that the OD value during dispersion evaluation is 3.0 or higher. (4) Conductive material composite particles according to (1) or (2) above, characterized in that the dispersant is a nonionic dispersant. (5) Conductive composite particles as described in (4) above, characterized in that the weight-average molecular weight of the nonionic dispersant is 1,000 to 1,000,000. (6) Conductive material composite particles according to (1) or (2) above, characterized in that the purity of the conductive material is 99.9% or higher. (7) Conductive material composite particles according to (1) or (2) above, characterized in that the average primary particle diameter of the conductive material is 10 nm or more and 50 nm or less. (8) Conductive material for battery electrodes, conductive material composite particles as described in (1) or (2) above,

[0010] (9) A method for manufacturing conductive composite particles, comprising a step of preparing a conductive material dispersion paste containing at least a conductive material, a dispersant, and a dispersion medium, and a step of removing the dispersion medium from the conductive material dispersion paste, wherein the particle diameter of the conductive composite particles in the conductive material dispersion paste is 50 μm or less. A method for manufacturing conductive composite particles, characterized in that (10) The method for manufacturing conductive composite particles according to (9) above, characterized in that the conductive material dispersion paste does not contain foreign matter having a particle diameter exceeding 50 μm. (11) The method for manufacturing conductive composite particles according to (9) or (10) above, characterized in that it includes a drying step of heating the conductive material dispersion paste at 80°C or higher and 300°C or lower. (12) A method for manufacturing an electrode, characterized in that the conductive composite particles according to (1) or (2) above are mixed with at least an active material and a binder and applied to a substrate. (13) A method for manufacturing an electrode, characterized in that the conductive composite particles obtained by the manufacturing method according to (9) or (10) above are mixed with at least an active material and a binder and applied to a substrate. (14) A lithium-ion secondary battery using the electrode obtained by the method according to (12) above. (15) An energy storage device using the conductive composite particles according to (1) or (2) above. (16) An energy storage device using the electrode obtained by the method according to (12) above. is in. [Effect of the Invention]

[0011] The conductive composite particles obtained by the present invention have a particle size distribution adjusted to a size at which secondary aggregation is easily broken up during the kneading process when preparing an electrode paste. Also, since they are particles containing a dispersant, they exhibit high dispersibility in a solvent. Therefore, since it is not necessary to knead for a long time when preparing an electrode paste, there is no risk of a decrease in conductivity due to re-aggregation caused by excessive fineness. Furthermore, the conductive composite particles of the present invention are likely to form a conductive path within the electrode coating film, so they are excellent in conductivity and can dramatically improve battery performance such as charge-discharge capacity. In addition, because coarse particles and foreign matter are removed, the conductive material is dispersed without being unevenly distributed within the paste and is uniformly distributed around the active material, providing sufficient electronic conductivity. This allows for the formation of an electrode coating that achieves excellent charge and discharge capacity. Furthermore, because localized current flow and short circuits caused by uneven distribution of coarse particles and conductive material are less likely to occur, it is believed that problems such as thermal runaway of the battery and premature deterioration of charge and discharge capacity can be suppressed, and battery performance can be maintained over the long term. Furthermore, since the conductive composite particles obtained by the present invention are a dry powder that is substantially free of solvents, there is no risk of deterioration over time due to sedimentation or solidification compared to liquid or paste-like conductive materials, and they have the stability to be stored at room temperature for a long period of time. In addition, the amount of organic solvents used during manufacturing can be reduced, thereby reducing the burden on the environment.

[0012] Therefore, the present invention makes it possible to produce conductive composite particles with excellent long-term stability while minimizing the burden on the environment. Furthermore, by using the conductive composite particles of the present invention as an electrode paste to produce positive and negative electrode plates, it is possible to manufacture lithium-ion secondary batteries with excellent battery performance, such as high charge / discharge capacity and long-term stability of charge / discharge capability. [Brief explanation of the drawing]

[0013] [Figure 1] Figure 1 is a flowchart illustrating the method for producing the conductive composite particles of the present invention. [Figure 2] Figure 2 shows a scanning electron microscope (SEM) image of a cross-section of the coating film prepared in Example 1. [Figure 3] Figure 3 shows an EDS (energy-dispersive X-ray spectrometer) analysis image of the cross-section of the coating film prepared in Example 1. [Figure 4] Figure 4 shows an SEM image of a cross-section of the coating film prepared in Example 2. [Figure 5] Figure 5 shows an EDS analysis image of the cross-section of the coating film prepared in Example 2. [Figure 6] Figure 6 shows an SEM image of a cross-section of the coating film prepared in Example 3. [Figure 7] Figure 7 shows an EDS analysis image of the cross-section of the coating film prepared in Example 3. [Figure 8] Figure 8 shows an SEM image of a cross-section of the coating film prepared in Example 4. [Figure 9] Figure 9 shows an EDS analysis image of the cross-section of the coating film prepared in Example 4. [Figure 10] Figure 10 shows an SEM image of a cross-section of the coating film prepared in Comparative Example 1. [Figure 11] Figure 11 shows an EDS analysis image of the cross-section of the coating film prepared in Comparative Example 1. [Figure 12] Figure 12 shows an SEM image of a cross-section of the coating film prepared in Comparative Example 2. [Figure 13] Figure 13 shows an EDS analysis image of the cross-section of the coating film prepared in Comparative Example 2. [Figure 14] Figure 14 shows an SEM image of a cross-section of the coating film prepared in Comparative Example 3. [Figure 15] Figure 15 shows an EDS analysis image of the cross-section of the coating film prepared in Comparative Example 3. [Figure 16] Figure 16 shows an SEM image of a cross-section of the coating film prepared in Comparative Example 4. [Figure 17] Figure 17 shows an EDS analysis image of the cross-section of the coating film prepared in Comparative Example 4. [Modes for carrying out the invention]

[0014] The conductive composite particles of the present invention refer to particles containing at least a conductive material and a dispersant. A detailed explanation follows below.

[0015] [Conductive materials] Suitable conductive materials for use in the present invention include carbon black, carbon nanotubes, carbon nanofibers, graphite, graphene, and hard carbon. Among these, carbon black is preferred because its structure efficiently forms conductive paths within the electrode, improving conductivity. As carbon black, Ketjenblack, furnace black, acetylene black, and thermal black can be used, but acetylene black is particularly preferred due to its high conductivity, low impurity content, and excellent heavy overload characteristics. These conductive materials can be used individually or in combination of two or more.

[0016] The average primary particle size of the conductive material is preferably between 10 nm and 50 nm, more preferably 45 nm or less, and even more preferably 40 nm or less. Furthermore, 10 nm or more is more preferable, and 15 nm or more is even more preferable. If the average primary particle size of the conductive material is greater than 50 nm, the conductivity of the coating obtained from the electrode paste may decrease. Also, if it is smaller than 10 nm, the viscosity of the conductive material dispersion paste and electrode paste will increase, which may make dispersion of the conductive material difficult depending on the equipment used, and may also worsen handling properties. In this context, the mean primary particle diameter refers to the arithmetic mean particle diameter measured using a transmission electron microscope, in accordance with ASTM:D3849-14.

[0017] The conductive material is characterized by having a DBP oil absorption rate of 550 ml / 100g or less. Preferably, it is 170 to 240 ml / 100g, and most preferably 170 to 230 ml / 100g. If the DBP oil absorption rate of the conductive material is greater than 550 ml / g, the viscosity of the conductive material dispersion paste and electrode paste will increase, making dispersion difficult and leading to uneven distribution of the conductive material, which may prevent it from exhibiting sufficient conductivity. The DBP oil absorption amount of conductive materials should be measured in accordance with JIS 6217-4.

[0018] The purity of the conductive material is preferably 99.9% or higher, and more preferably 99.95 to 100% by mass. By keeping the purity of the conductive material within the above range, it is possible to prevent battery short circuits caused by impurities and reduce the defect rate. The purity of carbon black can be calculated based on the amount of ash content, which is considered an impurity, as measured in accordance with JIS K1469 or JIS K6218.

[0019] Suitable conductive materials that meet the above conditions include, specifically, carbon black products such as Denka Black powder, Denka Black granules, Denka Black FX-35, Denka Black HS-100, Denka Black Li Li-100, Denka Black Li Li-250, Denka Black Li Li-400, Denka Black Li Li-435, etc. (all product names, manufactured by Denka Co., Ltd.), LITX 50, LITX 66, LITX 60R, LITX 200, LITX 300, LITX-HP, etc. (all product names, manufactured by Cabot Specialty Chemicals Inc.), SUPER P Li, C-NERGY SUPER C45, C-NERGY SUPER C65 (all product names, manufactured by Imerys Graphite & Carbon, Inc.). Preferably, the products are Denka Black powder, Denka Black granular, Denka Black FX-35, Denka Black HS-100, Denka Black Li Li-100, Denka Black Li Li-250, Denka Black Li Li-400, and Denka Black Li Li-435; more preferably, Denka Black granular, Denka Black FX-35, Denka Black Li Li-100, and Denka Black Li Li-435; and particularly preferably, Denka Black granular and Denka Black FX-35.

[0020] [Dispersant] In this invention, the term "dispersant" refers to an additive used to uniformly disperse inorganic and organic pigments in a dispersion medium and prepare a stable dispersion. Based on their ionic properties, dispersants are broadly classified into anionic, cationic, nonionic, and amphoteric types. Any dispersant can be used in this invention as long as it has the effect of loosening the aggregation of conductive materials, preventing the re-aggregation of conductive material particles, and uniformly distributing conductive material particles in the solvent. Among these, nonionic dispersants that do not have ionic functional groups are suitable because they do not inhibit the movement of lithium ions. As nonionic dispersants, those that act as a binder after film formation and do not affect electrical properties, or those with a low decomposition temperature that can be removed by heating during electrode fabrication, are preferably used. Dispersants with these characteristics include polyvinylpyrrolidone, polyvinyl butyral, polyvinylidene fluoride, polytetrafluoroethylene, polyhexafluoropropylene, polyvinyl alcohol, polyvinyl acetal, polyvinyl ether, polyether, polyhydric alcohol ester, cellulose acetate, cellulose acetate butyrate, methylcellulose, ethylcellulose, hydroxyethylcellulose, ethylhydroxyethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, and hydroxypropylmethylcellulose. Among these, methylcellulose or polyvinylpyrrolidone are the most preferred.

[0021] The nonionic dispersant preferably has a weight-average molecular weight of 1,000 to 1,000,000. More preferably, it is 5,000 to 300,000, and even more preferably 5,000 to 200,000. If the weight-average molecular weight exceeds 1,000,000, the viscosity of the conductive material dispersion paste increases, leading to a loss of fluidity and poor discharge, thus worsening handling. On the other hand, if the weight-average molecular weight falls below 1,000, the dispersibility is poor, making it difficult to manufacture the conductive material dispersion paste. These nonionic dispersants can be used individually or in combination of two or more types. Weight-average molecular weight can be measured using gel filtration chromatography (GPC). An example of measurement conditions for gel filtration chromatography is described below. Equipment: High-performance liquid chromatograph (Shimadzu Corporation, Prominence) Column: Shodex Corporation, OHpakSB-802.5HQ Shodex Corporation, OHpakSB-804HQ Detector: RI Eluent: 0.5M NaCl aqueous solution Flow rate: 1.0ml / min Sample concentration: 0.2 wt / vol% Column temperature: 40℃

[0022] [Method for manufacturing composite particles] The method for producing the composite particles of the present invention is not particularly limited, but as an example of a preferred embodiment disclosed herein, the above materials are mixed with a dispersion medium and an additive and stirred, and the resulting mixture is subjected to wet grinding to produce a conductive material dispersion paste, and the dispersion medium of the conductive material dispersion paste is removed by drying to produce conductive material composite particles. In this invention, the conductive material dispersion paste refers to a state in which the conductive material is crushed in a liquid dispersion medium and uniformly stabilized.

[0023] [Dispersion medium] The dispersion medium used in the conductive material dispersion paste is not particularly limited as long as it can disperse the conductive material and can be removed in the subsequent drying step, but it is preferable to use a dispersion medium that uniformly dissolves the dispersant used. Solvents that can uniformly dissolve the dispersant used in the present invention include water, methanol, ethanol, N-methyl-2-pyrrolidone, and methyl ethyl ketone. When used for lithium-ion secondary batteries, water or N-methyl-2-pyrrolidone is generally selected, but water is preferred from the viewpoint of safety, convenience in the subsequent drying step, and environmental considerations. The amount of dispersion medium is preferably in the range of 99.0 to 50.0% by mass, more preferably 99.0 to 60.0% by mass, and even more preferably 99.0% to 70.0% by mass. If the amount of dispersion medium is less than 50% by mass, the conductive material dispersion paste will have poor fluidity, and it may be difficult to achieve the desired maximum particle size and viscosity in the wet grinding process.

[0024] [Additives] When preparing conductive material dispersion paste, additives can be optionally selected and included as needed. Examples of additives include pH adjusters, binders, solvents, thickeners, defoamers, surfactants, preservatives, and antifungal agents. Any amount of these additives may be included, depending on the desired performance requirements of the electrodes and the amount of dispersion medium used, as long as it does not hinder the effects of the present invention.

[0025] Examples of pH adjusting agents include potassium hydroxide, sodium hydroxide, and triethanolamine. These pH adjusting agents may be included individually or in combination of two or more. The amount of pH adjusting agent can be adjusted as appropriate according to the desired pH.

[0026] Examples of binders include water-soluble polymers and emulsion resins. The binder may be natural, semi-synthetic, or synthetic. Specifically, cellulose, starch and their modified forms, natural rubber, rosin and its modified forms, polyvinyl alcohols, acrylic resins, epoxy resins, urethane resins, melamine resins, etc., may be used as binders. The above binders may be contained individually or in combination of two or more.

[0027] To adjust the drying properties and film-forming properties of the coating film, a solvent may be included as an additive. The solvent referred to here is separate from the dispersion medium used for the wet grinding process and refers to the following substances used as adjusting agents. Examples of solvents that may be used include alcohols, alkyl ether alcohols, glycols, and diols. Examples of alcohols include methanol, ethanol, and isopropyl alcohol. Examples of alkyl ether alcohols include ethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, and propylene glycol monobutyl ether. Examples of glycols include ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, and polyethylene glycol with a number average molecular weight of 2000 or less. Examples of diols include glycerin. The above solvents may be contained individually or in combination of two or more.

[0028] Examples of thickeners include natural polysaccharides and synthetic polymer thickeners. Examples of natural polysaccharides include guar gum, locust bean gum, galactomannan, pectin and its derivatives, psyllium pseudogum, tamarind gum, microbial xanthan gum, rheozan gum, ramzan gum, wella gum, duran gum, seaweed polysaccharides such as carrageenan, alginic acid and its derivatives, and resin polysaccharides such as taragaant gum, cellulose and its derivatives. Examples of synthetic polymer thickeners include polyacrylic acid and its crosslinked copolymers, polyvinyl alcohol, polyvinylpyrrolidone and its derivatives, polyvinyl methyl ether and its derivatives, polyether acrylic resins and silicone acrylic resins, in the form of aqueous surfactant emulsions, aqueous self-emulsifying emulsions, and aqueous core-shell emulsions. The above thickeners may be contained individually or in combination of two or more.

[0029] [Mixing and stirring of materials] First, weigh the dispersant so that its weight ratio to the conductive material is as described later, add it to the dispersion medium, and stir until it is completely dissolved. If additives are added, add them at the same time as the dispersant, in an amount that does not impair the function of each material, depending on the application. There are no particular limitations on the method of stirring the materials, and commercially available stirrers, kneaders, mixers, etc. can be used. Next, add the conductive material to this dispersant solution and stir to obtain a conductive material mixture.

[0030] [Wet grinding process] The conductive material mixture is subjected to a wet grinding treatment and ground and dispersed until it reaches a predetermined viscosity and maximum particle size to obtain a predetermined conductive material dispersion paste as described below. Commercially available wet grinding equipment and wet dispersion equipment can be used for the wet grinding treatment. Any equipment capable of grinding to a predetermined viscosity and maximum particle size is acceptable, and the type and method of such equipment are not limited. Examples of wet media dispersers such as ball mills, sand grinders, dyno mills, spike mills, DCP mills, basket mills, and paint conditioners, as well as media-less dispersers such as nanomizers, altimizers, ultrasonic dispersers, thin-film swirling high-speed mixers, roll mills, colloid mills, high-pressure dispersers, homogenizers, and in-line mixers can be selected.

[0031] The particle size of the carbon black in the conductive material dispersion paste is preferably 50 μm or less as the maximum particle size, preferably less than 50 μm, more preferably 40 μm or less, and particularly preferably 30 μm or less. If the maximum particle size of the paste exceeds 50 μm, the distribution of active material and conductive material in the electrode coating of the battery may become non-uniform, impairing battery performance. The maximum particle size should be measured using a grind gauge in accordance with JIS K5600-2-5.

[0032] The viscosity of the conductive material dispersion paste is preferably 3000 mPa·s or less, more preferably 2000 mPa·s or less, and even more preferably 1000 mPa·s or less. By keeping the viscosity of the paste within the above range, fluidity is improved, resulting in good handling properties such as liquid transfer and discharge.

[0033] [Foreign matter removal process] Since the measurement of the maximum particle size using the aforementioned grind gauge cannot completely remove particles larger than 50 μm, it is preferable to include a step to remove foreign matter and coarse particles larger than 50 μm after preparing the conductive material dispersion paste. Here, foreign matter refers to any substance containing conductive material larger than 50 μm that is included in the dispersion paste. By removing foreign matter and coarse particles, it is possible to prevent coarse particles from coming into contact with each other on the electrode coating when used as an electrode paste and forming a short circuit, thereby reducing the risk of internal short circuits occurring in the battery. The method of removing foreign matter is not particularly limited, but foreign matter larger than a predetermined size can be removed by transporting the conductive material dispersion paste using a liquid pump or the like and passing it through a filter or magnetic separator set up in the path.

[0034] [Dispersion medium removal process] The conductive material dispersion paste prepared by the above method is dried, and part or all of the dispersion medium is removed and atomized to produce the conductive material composite particles of the present invention. The method for removing the dispersion medium is not particularly limited, and commercially available dryers such as freeze dryers, spray dryers, airflow dryers, thermal dryers, and fluidized bed dryers can be used.

[0035] [Spray drying] Among these drying methods, spray drying is particularly preferable because it allows for simultaneous drying, particle formation, and particle size adjustment, offering advantages such as shortened processing times and simplified equipment. Various types of spray dryers can be used for spray drying, such as centrifugal spraying and spray atomizing, but the method is not particularly limited as long as it involves spraying a liquid or a mixture of solid and liquid into a gas to dry it. When using a spray dryer, the droplet size can be reduced by increasing the amount of conductive material dispersion paste supplied, the amount of compressed air supplied, and the rotation speed of the disk. Through this operation, the particle size after drying can be adjusted to a predetermined range described later, thereby obtaining the conductive material composite particles of the present invention.

[0036] [Temperature control during drying] When drying conductive material dispersion paste, increasing the heating temperature can speed up the drying process and further reduce the moisture content after drying. The heating temperature during drying can be finely adjusted from the temperature at which each material dries, but for all conductive materials, temperatures below 80°C tend to leave moisture in the powder after drying, and temperatures above 300°C tend to cause deterioration and decomposition of the material. Therefore, a temperature of 80-300°C is preferable, and 100-150°C is more preferable.

[0037] [Moisture content after drying] In removing the dispersion medium, it is preferable that the moisture content of the conductive material composite particles after drying be 4% by weight or less, and more preferably 1% by weight or less. Water and other dispersion media remaining in the conductive material composite particles reduce the solubility of polyvinylidene fluoride, which is most commonly used as a binder during electrode paste formation. This prevents the creation of a uniform coating film, leading to a decrease in the strength within the composite layer and a decrease in adhesion between the composite layer and the metal foil. Furthermore, during the initial charging of a lithium-ion secondary battery, the remaining water decomposes, and the resulting hydrogen and oxygen can degrade the components within the battery. If the moisture content exceeds 4% by weight, these effects become more significant, potentially reducing battery performance. After drying, the moisture content should be measured, and if a large amount of moisture remains, the drying time should be extended until the moisture content falls below a predetermined value. Moisture content can be measured using a commercially available moisture meter such as a halogen lamp heating type moisture meter (Shinko Denshi Co., Ltd., MA-120).

[0038] [Grinding process] If a drying method other than spray drying is used in the dispersion medium removal step, the dried conductive material dispersion paste is pulverized until a powder having a predetermined particle size, as described later, is obtained. The pulverization method is not particularly limited, and commonly used methods such as hammer mills, crushers, mixers, mortars, and ball mills can be suitably selected according to the equipment and production volume.

[0039] [Sifting] Conductive composite particles with a specific particle size range can be obtained by classifying the conductive material composite particles from which the dispersion medium has been removed by sieving. The sieving operation is not particularly limited, but it can be carried out as follows: In accordance with JIS8801-1, multiple sieves with different nominal mesh sizes are stacked in order, with the largest mesh size on top. The conductive composite particles are then placed in the top sieve, and shaken for 30 minutes using an electromagnetic sieve shaker (manufactured by Fritsch Japan Co., Ltd.) to classify the conductive composite particles according to their particle size. By removing the particles that passed through the desired mesh size and those that did not, conductive composite particles classified to a predetermined particle size can be obtained.

[0040] The conductive composite particles obtained through these operations have the following characteristics and desirable properties.

[0041] The conductive composite particles of the present invention are characterized by having a particle size distribution D50 of 15 μm or more. Preferably, it is 20 μm or more, and more preferably 25 μm or more. Our investigations revealed that when the particle size distribution D50 is smaller than 15 μm, the battery charge / discharge capacity and cycle characteristics decrease. We also found that when mixing with the active material to form an electrode paste, kneading for more than 10 minutes significantly impairs conductivity. While the mechanism is not fully clear, the inventors speculate as follows. As mentioned above, conductive materials contribute to improved conductivity and battery performance by connecting electrodes and active materials, as well as active materials themselves, within the electrode coating, thereby forming conductive paths. When mixing electrode paste, forces are also applied to the conductive material, but the effect of these forces differs depending on whether the particle size is large or small. Specifically, for example, if the conductive material is carbon black, carbon black generally forms a structure in which primary particles are linked by chemical bonds. These structures then strongly aggregate into secondary aggregates (aglomerates) ranging from tens to hundreds of microns in size through van der Waals bonds, simple adhesion, or entanglement. Within the conductive material composite particles, carbon black mainly exists in the state of secondary aggregates. When forces are applied to the composite particles during mixing in electrode paste preparation, these forces are naturally also applied to the secondary aggregates within the composite particles. However, in the case of large particles, most of the force is used to loosen the secondary aggregates, and less is used to further break the bonds between the structures after loosening the aggregates. Therefore, even after mixing, the bonds between the structures are maintained to some extent as they are dispersed in the paste, making it easier to form conductive paths and improving conductivity and battery performance. However, in the case of small particles, secondary aggregates do not develop significantly, and little force is required to break them up. Therefore, the force applied during mixing is largely used to further break the bonds between structures after the aggregates have been broken up. As a result, individual structures with broken bonds, or small aggregates with only a few structures bonded together, are dispersed within the paste, making it difficult to form conductive paths and preventing the development of high conductivity and battery performance.

[0042] Due to the effects described above, the bonding between structures is maintained even after the electrode paste is formed, and in addition to particle size distribution D50, particle size distribution D10, particle size distribution D90, and average particle size can also be used as indicators to show the range of particle sizes that can exhibit high conductivity and battery performance. When using particle size distribution D10 as an indicator, it is preferable that the particle size is 10 μm or larger, and particularly preferable that it is 15 μm or larger. When using particle size distribution D90 as an indicator, it is preferable that the particle size is 30 μm or larger, and particularly preferable that it is 40 μm or larger.

[0043] In this invention, particle size distributions D10, D50, and D90 refer to the particle diameters corresponding to which the cumulative volume, when plotted from the smallest diameter side, is 10% for D10, 50% for D50, and 90% for D90, based on the cumulative particle size distribution curve obtained by photographing a scattered group of particles using a scanning electron microscope and measuring the maximum diameter passing through the center point of each particle. The particle size distributions D10, D50, and D90 of conductive composite particles can be measured by the following methods, but the measurement method is not limited to these if the same results can be obtained. First, a conductive sample stage is used to prevent static charge buildup, and conductive composite particles are scattered on the sample stage so that the particles do not overlap. Next, a scanning electron microscope (Hitachi High-Tech Corporation, FlexSEM1000) and AZtec (Oxford Instruments Inc., particle analysis software) are used to analyze more than 300 particles, and the diameter can be calculated from the numerical values ​​obtained.

[0044] In the present invention, the conductive composite particles preferably have a particle size of 150 μm or less as determined by sieving. More preferably, the particle size as determined by sieving is 100 μm or less, and particularly preferably 75 μm or less. By reducing the particle size to 150 μm or less through sieving, coarse particles are removed, improving battery performance. If the particle size is larger than 150 μm through sieving, uneven distribution of conductive material occurs within the electrode coating, preventing sufficient electronic conductivity from being imparted to the active material. The particle size determined by sieving, as used herein, refers to the smallest mesh opening of a sieve conforming to JIS 8801-1 through which the target particle can pass. While the sieving operation is not particularly limited as long as the sieve conforms to JIS 8801-1, the method disclosed in 0039 allows the particle to pass through sieves of different sizes in a single measurement, enabling simultaneous measurement of particle size, classification of particles, and extraction of particles of the desired particle size.

[0045] In the present invention of the second application, the conductive material composite particles are characterized in that the upper limit of the particle size is 300 μm or less. Preferably, the upper limit of the particle size is 200 μm or less, and particularly preferably 100 μm or less. By setting the upper limit of the particle size to 300 μm or less, coarse particles are removed, and the composite particles can obtain sufficient dispersion power. As a result, when the electrode paste is formed and applied, the conductive material is uniformly dispersed in the paste and the electrode coating, sufficient conductivity can be imparted to the active material, and the charge and discharge capacity of the battery can be improved. If the upper limit of the particle size is greater than 300 μm, the composite particles are too large compared to the typical size of the active material, which is several μm to tens of μm, and the composite particles cannot obtain sufficient dispersion power during kneading when forming the electrode paste, resulting in uneven distribution of the conductive material in the electrode coating and insufficient electronic conductivity to be imparted to the active material. In this invention, the upper limit of particle size refers to the measured value obtained by the following method. However, the measurement method is not limited to this if the same result can be obtained. First, a conductive sample stage is used to prevent static charge buildup, and conductive composite particles are scattered on the sample stage so that the particles do not overlap. Next, a scanning electron microscope (Hitachi High-Tech Corporation, FlexSEM1000) and AZtec (Oxford Instruments Inc., particle analysis software) are used to measure the maximum diameter passing through the center point of approximately 300 particles, and the largest of these measured values ​​can be determined.

[0046] The conductive composite particles of the present invention preferably have an average particle diameter of 20 μm to 80 μm. More preferably, they are 25 μm to 60 μm or less, and most preferably 25 μm to 40 μm or less. By keeping the average particle diameter within the above range, uneven distribution of conductive material is less likely to occur, and handling is also improved. Furthermore, the aforementioned effect facilitates the formation of conductive paths, contributing to improved conductivity and battery performance. The average particle diameter referred to here is the arithmetic mean of the largest diameters passing through the center point of each particle, and can be calculated as follows, however, the measurement method is not limited to this as long as the same result is obtained. First, a conductive sample stage is used to prevent static charge buildup, and conductive composite particles are scattered on the sample stage so that the particles do not overlap. Next, a scanning electron microscope (Hitachi High-Tech Corporation, FlexSEM1000) and AZtec (Oxford Instruments Inc., particle analysis software) are used to analyze more than 300 particles, and the diameter can be calculated from the numerical values ​​obtained.

[0047] The conductive composite particles of the present invention may contain 9% or more particles with an aspect ratio of less than 1.2. Such conductive composite particles have superior handling properties during powdering and storage, and superior dispersibility when formed into a conductive paste, compared to those containing less than 9% of particles with an aspect ratio of less than 1.2. Furthermore, they may also contain 55% or more particles with an aspect ratio of less than 1.2 and 85% or more particles with an aspect ratio of less than 1.5. Particles containing the above-mentioned ratios or higher of particles with an aspect ratio of less than 1.2 and less than 1.5 are extremely preferable because they offer particularly improved dispersibility when formed into an electrode paste, as well as improved handling properties during powdering and storage. In particular, by forming the particles by the aforementioned spray drying method, such conductive composite particles with an aspect ratio close to 1 and particularly excellent handling properties and dispersibility can be obtained. The aspect ratio of the conductive composite particles of the present invention refers to the ratio (a / b) of the maximum diameter (a) and the minimum diameter (b) passing through the center point of each particle, and can be determined as follows, but the measurement method is not limited to this as long as the same result can be obtained. First, a conductive sample stage is used to prevent static charge buildup, and conductive composite particles are scattered on the sample stage so that the particles do not overlap. Next, the aspect ratio of each particle is measured using a scanning electron microscope (Hitachi High-Tech Corporation, FlexSEM1000) and AZtec (Oxford Instruments Inc., particle analysis software), and the aspect ratio can be calculated from the measurements of more than 300 particles. By using the aforementioned materials and manufacturing methods, the aspect ratio of the conductive composite particles can be kept within the above range. In particular, by using spray drying during the drying process, it is possible to produce a larger number of particles with an aspect ratio close to 1.

[0048] The conductive composite particles of the present invention preferably have an angle of repose of less than 45°. More preferably, the angle of repose is less than 38°. By keeping the angle of repose within the above range, good fluidity is achieved, and crosslinking does not occur during storage, eliminating the need for stirring or shaking before use, thus providing excellent handling during manufacturing and use. The angle of repose referred to here can be determined by following the measurement method specified in the Japanese Industrial Standard JIS 9301-2-:2:1999, which involves gently depositing powder on a horizontal surface and reading the angle between the naturally formed slope and the horizontal plane. However, the measurement method is not limited to this, as long as the same result can be obtained.

[0049] The conductive material composite particles are characterized by containing 1 to 10 parts by weight of dispersant per 100 parts by weight of conductive material. More preferably, they contain 4 to 10 parts by weight of dispersant, and most preferably 6 to 10 parts by weight. By using a dispersant, the conductive material can be uniformly crushed and dispersed in the dispersion medium during the wet grinding process. In addition, the presence of dispersant in the conductive material composite particles after drying improves their dispersibility in the solvent. If the amount of dispersant is less than 1 part by weight, there is a problem that sufficient dispersion effect in the dispersion medium cannot be obtained during the wet grinding process. Also, if the amount of dispersant exceeds 10 parts by weight, the resistance value increases, which has the problem of adversely affecting battery performance when used as an electrode coating. With the manufacturing method disclosed above, the decomposition temperature of the dispersant is not exceeded, and even after material mixing, wet dispersion, dispersion medium removal, and grinding steps, the dispersant is hardly removed, or is removed while being coated with the conductive material, thus maintaining its ratio to the conductive material. Therefore, by keeping the weight ratio of the dispersant added during material mixing and stirring within the aforementioned range, the weight ratio in the conductive material composite particles can also be kept within that range.

[0050] However, the dispersant ratio may vary depending on the equipment used and the type of dispersant. Therefore, the dispersant content in the conductive composite particles after fabrication can be measured and confirmed using the following method as needed. The weight of the dispersant can be determined from the difference in weight before and after heating by heating the conductive material composite particles for a time at which the dispersant can be completely decomposed at a temperature at which the dispersant decomposes thermally without changing the weight of the conductive material. Note that the weight of the dispersant is calculated based on the amount of the active ingredient if the dispersant is dissolved or dispersed in another solvent. The content ratio of each component in the composite particles can be easily determined by continuously heating the material in a temperature range where the dispersant decomposes but the conductive material does not, using a TG-DTA (thermogravimetric differential thermal analyzer), and measuring the thermogravimetric change of the dispersant. As a specific example, using TG-DTA (manufactured by Bruker Japan Co., Ltd.), conductive composite particles can be measured by holding them at 100°C for 1 hour to completely remove moisture, then raising the temperature to 300°C at a rate of 5°C per minute, and holding them for another hour to decompose the dispersant. The weight after holding at 300°C is the weight of the conductive material alone, and the difference between the weight after holding at 100°C and the weight after holding at 300°C is the amount of dispersant in the conductive composite particles.

[0051] The bulk density of the conductive composite particles is preferably 0.04 g / ml or higher. More preferably 0.10 g / ml or higher, and even more preferably 0.20 g / ml or higher. By setting the bulk density to the above values ​​or higher, the particles are less likely to scatter within the production plant, eliminating the need for dust collection equipment, and thus reducing transportation and storage costs. The bulk density can be measured by determining the initial bulk density in accordance with R-1627-1997.

[0052] As an evaluation of the dispersibility of conductive material composite particles, it is preferable that the arithmetic mean roughness (Ra) when a coating is formed is 0.4 or less. More preferably, it is 0.3 or less, and even more preferably, 0.2 or less. Arithmetic mean roughness (Ra) is a value calculated by measuring the height direction irregularities of a surface in a certain reference length (interval), using the average value as the reference line, and calculating the average of the absolute values ​​of the distances from the reference line within that interval. A high value of arithmetic mean roughness (Ra) means that there are many irregularities, that is, there is a high possibility that aggregation or uneven distribution of conductive material, coarse particles, and foreign matter, which are the main causes of irregularities on the coating film, are present. If the arithmetic mean roughness is greater than 0.4, these effects become larger, and uniform dispersion of the conductive material in the electrode coating film may be hindered. In this invention, the arithmetic mean roughness refers to a numerical value determined by the following method, but the method is not limited to this if the same result can be obtained. First, a commercially available polyvinylidene fluoride binder (Kureha Corporation, "KF Polymer W#7200") is diluted to 8.0% with the solvent N-methyl-2-pyrrolidone to prepare a paste. Conductive composite particles are weighed and added to this solution so that they make up 8.3% of the total, and the mixture is kneaded at 2000 rpm for 1 minute in a rotary-orbit mixer (Thinky Co., Ltd., Awatori Rentaro). This paste is applied to a glass plate using an applicator so that the film thickness after drying is 7.5 to 8.5 μm, and the film is dried in a 100°C hot air dryer for 30 minutes to remove the solvent and obtain a coating film. The obtained coating film is designated as the "dispersibility evaluation coating film". The surface roughness of the dispersibility evaluation coating film is measured using a contact-type surface roughness meter (Tokyo Seimitsu Co., Ltd., Surfcom130A), and the arithmetic mean roughness is calculated.

[0053] In the third invention of this application, the conductive composite particles are characterized in that the optical density (OD) value during dispersion evaluation is 3.0 or higher. More preferably, it is 3.5 or higher, and even more preferably, 4.0 or higher. The OD value during dispersion evaluation referred to here is a value obtained by measuring the aforementioned dispersibility evaluation coating film using a spectrophotometer (X-Rite, manufactured by VideoJet X-Rite Corporation), but the measurement method is not limited to this as long as the same result can be obtained. The OD value is a logarithmic representation of the degree of light absorption by measuring the spectral transmittance of a coating film. It is known that the value increases as less light passes through the coating film. The inventors' research has shown a correlation between the OD value and electrical properties, and that by adjusting the OD value of conductive composite particles when coated into a film to a predetermined range, excellent performance as a battery material can be obtained. More specifically, it was found that conductive composite particles with an OD value of 3.0 or higher when coated into a film exhibit uniform dispersion of the conductive material when coated into an electrode, resulting in increased charge and discharge capacity. Furthermore, it was found that when conductive composite particles with an OD value of less than 3.0 when coated into a film are used to create a battery, the uniformity of the dispersion of the conductive material tends to be low, and the charge and discharge capacity tends to decrease. The following reasons are possible for this: If the dispersion uniformity of the conductive material is insufficient, and a large amount of conductive material aggregates and is unevenly distributed within the coating, then areas where conductive material is absent will occur unevenly. Since these areas do not absorb light, the amount of light transmitted through the entire coating increases, resulting in a decrease in the OD value. The fact that the conductive material is not uniformly dispersed within the coating means that sufficient electronic conductivity cannot be imparted to the active material, leading to a decrease in the charge and discharge capacity when the device is made into a battery, and a premature decrease in capacity due to localized current flow. Therefore, a decrease in the OD value can be considered to indicate that the dispersion of the conductive material is insufficient and that the performance when the device is made into a battery is reduced.

[0054] The conductive composite particles of the present invention, satisfying the above properties, exhibit the following performance and effects. Compared to conductive materials in powder or granular form that do not contain dispersants, etc., the shear viscosity when formed into an electrode paste is low, resulting in excellent coating properties. The appropriate viscosity gradient between low and high shear suppresses the settling of the active material and prevents uneven distribution of the conductive material when a coating film is formed. Furthermore, because the aggregate is easily broken down during the mixing process when forming the electrode paste, the film thickness becomes uniform when a coating film is formed, making it less likely for areas with thin film thickness or areas where no coating film is formed to occur, thus increasing the yield. Moreover, when forming the electrode paste, the paste becomes uniform with a shorter stirring time compared to conductive materials in powder or granular form, resulting in cost benefits due to reduced process time. In addition, compared to conventional conductive composite particles, the conductive material is less likely to be unevenly distributed within the coating film. This allows the conductive material to be more uniformly dispersed among more active materials within the electrode coating film, imparting electronic conductivity and increasing the charge / discharge capacity. Furthermore, it is believed that this reduces the occurrence of premature capacity degradation due to localized current concentration at the positive and negative electrodes. Furthermore, because the conductive material composite particles do not contain solvents, the amount of organic solvent used during manufacturing can be reduced compared to slurry-type conductive material dispersion pastes, thereby reducing the environmental burden. In addition, it can be used regardless of the type or presence of solvents used when forming the electrode paste.

[0055] [Method of manufacturing lithium-ion secondary batteries] The lithium-ion secondary battery of the present invention can be manufactured by winding a positive electrode, prepared by dispersing the conductive composite particles of the present invention together with an electrode active material and a binder in a non-aqueous solvent such as N-methyl-2-pyrrolidone, onto a metal substrate and drying the electrode paste, or by dispersing the conductive composite particles of the present invention together with an electrode active material and a binder in a solvent such as water, onto a metal substrate and drying the electrode paste, onto a separator and housing the positive electrode, onto the separator and a negative electrode, onto the separator and a battery case together with an electrolyte. To prevent pressure rise inside the battery, overcharge and overdischarge, etc., overcurrent prevention elements such as fuses and PTC elements, expanded metal, lead plates, etc., may be provided as needed. The shape of the battery case may be, for example, coin-shaped, button-shaped, sheet-shaped, cylindrical, rectangular, flat, etc.

[0056] [Method of use in other energy storage devices] The characteristics of the conductive composite particles of the present invention, such as uniformity during dispersion and substantially no solvent content, are effective not only in lithium-ion secondary batteries but also in other energy storage devices, contributing to improved charge / discharge performance, cost reduction, and reduced environmental impact. In particular, in energy storage devices that use carbon materials such as carbon black, carbon nanotubes, and graphite, or slurries using them, as conductive materials, the conductive composite particles of the present invention can be suitably used as a substitute for these existing conductive materials. For example, it can be suitably used in primary batteries such as alkaline manganese dry cells and nickel manganese batteries, secondary batteries such as nickel-metal hydride batteries, nickel-cadmium batteries, sodium-sulfur batteries, and sodium-ion batteries, as well as capacitors and other electrochemical elements. When used in these energy storage devices, the existing conductive material can be replaced with the conductive material composite particles of the present invention. If the existing conductive material is a granular conductive material such as carbon black, it can be replaced directly. If the conductive material is a slurry such as a carbon black dispersion, it can be used by replacing the conductive material with the conductive material composite particles of the present invention, adjusting the solid content of the electrode paste, and achieving an appropriate viscosity. [Examples]

[0057] The present invention will be described in more detail below with reference to examples.

[0058] [Example 1] To 84 parts by weight of ion-exchanged water used as a dispersion medium, 1 part by weight of methylcellulose polymer (weight-average molecular weight of 35,000 as measured by GPC) was added as a dispersant, and the mixture was thoroughly dissolved using a commercially available stirrer to obtain a dispersant solution. Next, to 85 parts by weight of the obtained dispersant solution, 15 parts by weight of acetylene black (manufactured by Denka Co., Ltd., "Denka Black Granules"), which has an average primary particle size of 35 nm as measured according to the ASTM:D3849-14 method, was added as a conductive material, and the mixture was stirred using a commercially available stirrer to obtain a conductive material mixture. The obtained conductive material mixture was subjected to wet grinding using a commercially available horizontal bead mill until particles larger than 50 μm were eliminated as evaluated by a grind gauge, thereby refining the conductive material to obtain a conductive material dispersion paste. The obtained conductive material dispersion paste was filtered through a 50 μm mesh to remove foreign matter, and was obtained as conductive material dispersion paste 1. The conductive material dispersion paste 1 was dried using a commercially available spray nozzle type spray dryer at an inlet temperature of 160°C and a spray pressure of 0.03 MPa. Conductive material composite particles 1 with particle sizes of 75 μm or less and 45 μm or more were obtained by sieving using an electromagnetic sieve shaker (manufactured by Fritsch Japan Co., Ltd.).

[0059] [Example 2] To 84 parts by weight of ion-exchanged water used as a dispersion medium, 1 part by weight of polyvinylpyrrolidone (weight-average molecular weight 66,800 as measured by GPC) was added as a dispersant, and the mixture was thoroughly dissolved using a commercially available stirrer. The same procedure as in Example 1 was then carried out to obtain conductive material dispersion paste 2. Aside from using this conductive material dispersion paste 2, the subsequent operations were carried out in the same manner as in Example 1, and conductive material composite particles 2 with particle sizes of 75 μm or less and 45 μm or more were obtained by sieving using an electromagnetic sieve shaker (manufactured by Fritsch Japan Co., Ltd.).

[0060] [Example 3] Conductive material dispersion paste 1 obtained by performing the same procedure as in Example 1 was dried at 100°C for 12 hours using a commercially available hot air dryer. The resulting dried material was then pulverized using a commercially available cutter mixer and classified using an electromagnetic sieve shaker (manufactured by Fritsch Japan Co., Ltd.) to obtain conductive material composite particles 3 with particle sizes of 75 μm or less and 45 μm or more, as determined by sieving.

[0061] [Example 4] Conductive material dispersion paste 1 obtained by performing the same procedure as in Example 1 was dried at 100°C for 12 hours using a commercially available hot air dryer. The resulting dried material was then pulverized using a commercially available cutter mixer and classified using an electromagnetic sieve shaker (manufactured by Fritsch Japan Co., Ltd.) to obtain conductive material composite particles 4 with particle sizes of 150 μm or less and 75 μm or more, as determined by sieving.

[0062] [Comparative Example 1] Conductive material dispersion paste 1 obtained by performing the same procedure as in Example 1 was dried at 100°C for 12 hours using a commercially available hot air dryer. The resulting dried material was then pulverized using a commercially available cutter mixer and classified using an electromagnetic sieve shaker (manufactured by Fritsch Japan Co., Ltd.) to obtain conductive material composite particles 5 with particle sizes of 250 μm or less and 150 μm or more, as determined by sieving.

[0063] [Comparative Example 2] Conductive material dispersion paste 1 obtained by performing the same procedure as in Example 1 was dried at 100°C for 12 hours using a commercially available hot air dryer. The resulting dried material was then pulverized using a commercially available cutter mixer and classified using an electromagnetic sieve shaker (manufactured by Fritsch Japan Co., Ltd.) to obtain conductive material composite particles 6 with particle sizes of 500 μm or less and 250 μm or more, as determined by sieving.

[0064] [Comparative Example 3] Conductive material dispersion paste 1 obtained by performing the same procedure as in Example 1 was dried at 100°C for 12 hours using a commercially available hot air dryer. The resulting dried material was then pulverized using a commercially available cutter mixer and classified using an electromagnetic sieve shaker (manufactured by Fritsch Japan Co., Ltd.) to obtain conductive material composite particles 7 with a particle size of 45 μm or less by sieving.

[0065] [Comparative Example 4] Commercially available acetylene black (Denka Co., Ltd., "Denka Black Powder") was used as conductive material 1. This acetylene black is the same type as the acetylene black used as the conductive material in each example and comparative example.

[0066] [Physical property measurement and performance evaluation] The dispersion pastes, conductive composite particles, and conductive raw materials prepared by the methods disclosed in each of the above examples and comparative examples were subjected to physical property measurements and performance evaluations by the following procedures.

[0067] The DBP oil absorption amount of acetylene black used as a conductive material in each example and comparative example was measured according to the method in accordance with JIS 6217-4. The measured value was 360 ml / 100 g. The viscosity of conductive material dispersion pastes 1 and 2 was measured using a B-type viscometer (TVB10M viscometer, manufactured by Toki Sangyo Co., Ltd.). The moisture content of conductive material composite particles 1, 2, 3, 4, 5, 6, and 7, and conductive material raw material 1 was measured using a halogen lamp heated moisture meter (Shinko Denshi Co., Ltd., MA-120). For conductive composite particles 1, 2, 3, 4, 5, 6, and 7, the upper limit of particle size, average particle size, lower limit of particle size, particle size distributions D10, D50, D90, D95, and aspect ratio were measured and calculated using a scanning electron microscope (Hitachi High-Tech Corporation, FlexSEM1000) and AZtec (Oxford Instruments Inc., particle analysis software). The angle of repose was also measured in accordance with JIS 9301-2-:2:1999. The measurement results are shown in Table 1.

[0068] The arithmetic mean roughness of conductive composite particles 1, 2, 3, 4, 5, 6, and 7 and conductive raw material 1 was measured using the following method. A commercially available binder, polyvinylidene fluoride (Kureha Corporation, "KF Polymer W#7200"), was diluted to 8.0% with the solvent N-methyl-2-pyrrolidone. Conductive composite particles or conductive raw materials were weighed and mixed in such a solution that they comprised 8.3% of the total, and the mixture was kneaded at 2000 rpm for 1 minute in a rotary-orbit mixer (Thinky Co., Ltd., Awatori Rentaro) to prepare a paste. This paste was applied to a glass plate using an applicator to achieve a film thickness of 7.5 to 8.5 μm after drying, and the film was dried in a 100°C hot air dryer for 30 minutes to remove the solvent and obtain a coating film. The coating obtained here was designated as the "coating for dispersibility evaluation." The arithmetic mean roughness (Ra) and arithmetic mean height (Sa) of this coating were measured and calculated using a contact-type surface roughness meter (Surfcom130A, manufactured by Tokyo Seimitsu Co., Ltd.). Furthermore, the OD value of the coating film used for dispersibility evaluation was measured using a spectrophotometer (X-Rite, manufactured by VideoJet X-Rite Corporation).

[0069] For conductive composite particles 1, 2, 3, 4, 5, 6, 7 and conductive material raw material 1 (let's call it A), the commercially available active material is Li Ni 1 / 3 Co o 1 / 3 M n 1 / 3 The composition of O2, HED NCM111 1100 (manufactured by BASF Toda Battery Materials LLC) (referred to as B), and PVdF (Solef5130 manufactured by Solvay Japan) (referred to as C) were weighed out in the following order based on the weight ratio of their respective solid contents: A = 2 parts by weight, B = 97 parts by weight, and C = 1 part by weight. After diluting A, B, and C with NMP so that the total solid content of A, B, and C was 65% by mass, the mixture was kneaded in a rotary-orbit mixer at 2000 rpm for 2 minutes to obtain "Evaluation Electrode Paste A". "Evaluation Electrode Paste B" was obtained in the same manner as for Coating Film Evaluation Paste A, except that the kneading time in the rotary-orbit mixer was changed to 10 minutes. For conductive composite particles 1, 2, 3, 4, 5, 6, and 7, the dispersant contained in the composite particles acts as a binder on the electrode coating. Therefore, the amount of dispersant is considered as the amount of binder, and in calculating the mixing ratio, the amount of dispersant contained in the conductive composite particles is divided from A, and this amount of dispersant is added to C. Then, the conductive composite particles, active material, and binder were weighed so that A, B, and C were 2 parts by weight, 97 parts by weight, and 1 part by weight, respectively. The above-mentioned evaluation electrode pastes A and B were applied onto a PET film using an applicator, and then dried in a hot air dryer at 100°C for 30 minutes to remove NMP, resulting in a coating film with a thickness in the range of 100 μm to 120 μm. The coating films obtained here are referred to as "Electrode Paste Evaluation Coating Film A" and "Electrode Paste Evaluation Coating Film B," respectively. Of these, coating film A for electrode paste evaluation was cross-sectioned using a cross-sectional cutter for SEM sample preparation, and the dispersion state of the conductive material in the coating film was imaged at a magnification of 500x using a scanning electron microscope (Hitachi High-Tech Corporation, FlexSEM1000) and an energy-dispersive X-ray spectrometer (Oxford Instruments Inc., AZtecEnergy x-act). Furthermore, the coating resistance was measured using both coating films A and B for electrode paste evaluation by the following method. First, the coating film was cut into strips 2 cm wide and 3 cm long, and its volume resistivity was measured using a resistivity meter (Loresta-GP MCP-T610, manufactured by Nitto Seikou Analytech Co., Ltd.) at an applied voltage of 10 V. The results of each of the above measurement tests are shown in Table 1.

[0070] [Battery performance evaluation] Next, using conductive composite particles 1, 2, 3, 4, 5, 6, and 7, conductive raw material 1, and the following materials, a CR2032 type (20 mm in diameter, 3.2 mm in height) coin-type secondary battery was fabricated using the following method, and its performance was evaluated. The secondary battery and its manufacturing method disclosed below are merely examples for evaluating the conductive composite particles of the present invention and do not limit the methods of use or embodiments of the conductive composite particles of the present invention. The conductive composite particles of the present invention can be suitably used as electrode materials for a wide range of energy storage devices, including lithium-ion secondary batteries, without being limited to the materials, battery manufacturing methods, battery forms, and other conditions disclosed below.

[0071] [Fabrication of the positive electrode in Example 1] The positive electrode active material (HED NCM111 1100, manufactured by BASF Toda Battery Materials LLC, theoretical capacity: 160 mAh / g), binder (PVdF (Solvay Japan, Solef5130)), and conductive material (conductive material composite particles 1) were weighed in a ratio of 97:1:2 (by weight). The binder was dissolved in a solvent (N-methyl-2-pyrrolidone), to which the positive electrode active material and conductive material were mixed. The mixture was then diluted with the solvent to a solid content of 70-75% by mass of the coating solution, forming an electrode paste. This electrode paste was then applied to a 20 μm thick aluminum foil with a basis capacity (battery capacity per unit area) of 3.0-4.0 mAh / cm². 2The coating was applied in this manner and dried in a 100°C oven for 30 minutes to obtain a positive electrode coating plate. In the positive electrode coating plate obtained here, the portion on the aluminum foil, which is the current collector, where the electrode paste has dried and formed is defined as the "positive electrode composite layer". This positive electrode coating plate has an electrode density (mass of positive electrode composite layer / volume of positive electrode composite layer) of 2.9 to 3.4 g / cm³. 3 After pressing it in that manner, it was punched out to a diameter of Φ14mm using a coin-type punching machine to form the positive electrode. In calculating the electrode density, the mass of the positive electrode composite layer was obtained by subtracting the mass of the aluminum foil from the mass of the positive electrode coating plate, and the volume of the positive electrode composite layer was calculated by subtracting the thickness of the aluminum foil from the thickness of the positive electrode coating plate to obtain the height, and then using a base area of ​​Φ14 mm.

[0072] [Preparation and evaluation of the evaluation battery for Example 1] As described above, a separator ("Seillon P2010", material: polypropylene, Φ17mm, manufactured by CS Tech) was inserted between the positive electrode and the negative electrode (lithium foil, thickness 200 μm, φ16 mm, manufactured by Honjo Metal Co., Ltd.), and a CR2032 type coin cell was assembled by filling it with electrolyte (1.0 M LiPF6 EC : DEC (1:1 v / v%), manufactured by Kishida Chemical Co., Ltd.) to produce an evaluation battery. The obtained evaluation batteries were subjected to the following charge-discharge and actual discharge capacity measurements using a charge-discharge tester (Model 580, High-Performance Charge-Discharge System, manufactured by Scribner Associates). The charge and discharge tests were performed at room temperature (25°C) using a constant current / voltage charge of 1C to fully charge the battery to a maximum voltage of 4.3V, and then discharged to 3.0V using a constant current of 1C. This constituted one cycle, and a total of 30 cycles were performed. The actual discharge capacity at the 30th cycle was measured. A 10-minute rest period was included between the completion of full charge and the start of discharge in each cycle, and between each cycle. Based on the obtained actual discharge capacity (mAh), the initial effective capacity, effective capacity at the 30th cycle, and capacity retention rate were determined. The results are shown in Table 1. Here, effective capacity refers to the ratio of the actual discharge capacity to the theoretical capacity (mass of the positive electrode composite layer (g) × active material blending ratio × theoretical capacity of active material per gram), which is set to 100%. Furthermore, the effective capacity of the first cycle was defined as the initial effective capacity. Furthermore, the capacity retention rate is a value that shows the ratio of the effective capacity at the 30th cycle to the initial effective capacity, which is set at 100%.

[0073] [Battery fabrication and evaluation of Examples 2-4 and Comparative Examples 1-4] The conductive material was changed to the composite particles and conductive material raw materials shown in Table 1, and a battery was fabricated in the same manner as in Example 1, and its performance was evaluated. The results are shown in Table 1.

[0074] [Table 1]

[0075] As shown in Table 1, the conductive composite particles in the examples had an OD value of 3.0 or higher after coating, indicating that the conductive material was uniformly dispersed in the coating. In particular, Example 1 showed excellent uniformity, with an optical density of over 4.0 in the coating. Comparing Examples 3 and 4, which differ only in their upper particle size limit, with Comparative Examples 1 and 2, the comparative examples with larger composite particle sizes show a decrease in optical density, indicating a decrease in uniformity. Furthermore, Comparative Examples 1 and 2 have rougher surfaces compared to the examples, indicating the presence of many coarse particles in the coating film. Comparative Example 4 has a low OD value and poor surface roughness of the coating film.

[0076] In the SEM images of Figures 2, 4, 6, 8, 10, 12, 14, and 16, the white spherical objects are the active material, and the gray haze surrounding them indicates the conductive material. In the EDS analysis images shown in Figures 3, 5, 7, 9, 11, 13, 15, and 17, the conductive material, carbon, is visualized as white. These figures show that in the examples, the conductive material is evenly and uniformly distributed among the active materials, whereas in the comparative examples, there are areas where it is concentrated and areas where it is almost absent, indicating uneven distribution within the coating film. Furthermore, the EDS analysis images of Comparative Examples 1, 2, and 4 show the presence of coarse particles or aggregates (white clumps in the images). In Figure 15, no large clumps are visible, but there is also almost no white haze. This suggests that the secondary aggregates were excessively loosened during the paste mixing process, and the bonds between the structures were broken. In such a state, conductive paths are difficult to form, and high battery performance cannot be achieved. Thus, it can be seen that by using the composite particles prepared according to the present invention, it is possible to obtain an electrode coating film with better dispersibility and uniform dispersion of conductive material compared to the comparative example.

[0077] In the evaluation of volume resistance, the electrode coatings using conductive composite particles in each example were shown to have generally lower resistance compared to the comparative example. In Comparative Example 3, the resistance was somewhat low when the mixing time during paste formation was 2 minutes, but the resistance increased significantly when it was 10 minutes. This is thought to be because, compared to the examples, there are many smaller composite particles, making it easier for the bonds between structures to break during mixing and making it difficult to form conductive paths.

[0078] In the evaluation of battery performance, the batteries using conductive composite particles in each example showed high battery performance. Comparative Examples 1, 2, and 3 had low effective capacity and capacity retention rates. In addition, Comparative Example 4 showed a significant decrease in capacity retention rate after 30 cycles.

[0079] As described above, in the electrode coating film using the conductive composite particles of the present invention shown in the examples, the conductive material is uniformly dispersed among the active materials in the coating film, allowing sufficient electronic conductivity to be imparted to many active materials, thereby improving the charge and discharge capacity when the device is made into a battery. Furthermore, since there is no uneven distribution or aggregation of the conductive material, and no coarse particles or foreign matter are present, the possibility of localized current flow or short circuits causing thermal runaway of the battery or premature degradation of charge and discharge capacity is lower compared to conventional technology, and battery performance can be maintained over the long term. [Industrial applicability]

[0080] By using the conductive composite particles of the present invention, high-quality positive and negative electrodes that improve the performance of lithium-ion secondary batteries can be manufactured. Lithium-ion secondary batteries using the present invention can be suitably used as power sources for electric motors installed in electric vehicles and the like. In addition, the conductive composite particles of the present invention can be stored at room temperature for a long period of time, and the amount of solvent used during manufacturing is reduced, thereby reducing the environmental burden and manufacturing costs. Furthermore, by using the conductive composite particles of the present invention as a substitute for existing conductive materials such as carbon black, it can contribute to improved performance and cost reduction not only in lithium-ion secondary batteries but also in other energy storage devices.

Claims

1. Conductive composite particles (excluding those that do not contain particles with a particle size of less than 50 μm as determined by sieving) characterized by having at least a conductive material and a dispersant, a particle size distribution D50 of 15 μm or more and a particle diameter of 150 μm or less as determined by sieving, a DBP oil absorption amount of the conductive material of 550 ml / 100 g or less, and containing 1 to 10 parts by weight of dispersant per 100 parts by weight of conductive material.

2. Conductive composite particles characterized by containing at least a conductive material and a dispersant, wherein the particle size distribution D50 is 15 μm or more and the upper limit of the particle diameter is 300 μm or less, the DBP oil absorption amount of the conductive material is 550 ml / 100 g or less, and the amount of dispersant is 1 to 10 parts by weight per 100 parts by weight of conductive material (excluding those that do not contain particles with a particle diameter of less than 50 μm as determined by sieving).

3. The conductive material composite particle according to either claim 1 or 2, characterized in that it contains at least a conductive material and a dispersant.

4. The conductive composite particle according to claim 1 or 2, characterized in that the dispersant is a nonionic dispersant.

5. The conductive composite particle according to claim 4, characterized in that the weight-average molecular weight of the nonionic dispersant is 1,000 or more and 1,000,000 or less.

6. The conductive material composite particle according to claim 1 or 2, characterized in that the purity of the conductive material is 99.9% or higher.

7. The conductive material composite particle according to claim 1 or 2, characterized in that the average primary particle diameter of the conductive material is 10 nm or more and 50 nm or less.

8. A conductive material composite particle according to claim 1 or 2, which is a conductive material for battery electrodes.

9. A method for manufacturing an electrode, characterized by mixing conductive composite particles according to claim 1 or 2 with at least an active material and a binder, and applying the mixture to a substrate.

10. A lithium-ion secondary battery using an electrode obtained by the method described in claim 9.

11. An energy storage device using conductive composite particles according to claim 1 or 2.

12. A power storage device using electrodes obtained by the method described in claim 9.

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